Boiler Sizing and Heat Load/Loss

I was reading some other posts and wanted to see how "over" sized my boiler is and what my real heat loss is. I am familiar with Manual J, but have never fully completed one for my house. I kept seeing mention of using historical Degree Day Data and Past Fuel Usage in other posts. I gathered mine to identify where I stand.

Background: House is 100+ years old, 3story colonial, with cast iron baseboard and radiators. Zoned into 8 heating zones. Heat is nat gas with a grossly oversized conventional boiler 275k BTU. It has an intellicon to help with my microzones and to prevent short cycling and aide in efficiency.

I used degree day data from the web and also from my gas company bill. Both appear very close. Can someone help interpret the data and verify my calc are correct? I did not adjust the BTU for the efficiency of my boiler, since I don't know if it is 70%, 80% etc...

The way I read this is my heat loss is 20k BTU/Hr??? is that right? Sounds way too low, or am I crazy or wrong?

Pink area is summer non heating months where gas usage was for cooking and DHW only.

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By using unadjusted source-fuel BTU data it will skew the calc to the high side, but you're a few orders of magnitude too low on the BTUs/CCF- 1 CCF= 102,000 BTU, not 1,020 BTU (But whats 100:1 between friends? )

And you don't say what your heating outside design temperature is, (which will vary depending on where you are in CT), nor have you calculated the fuel use rate at that temp.

Taking your January + February fuel usage only (since it's less contaminated with hot water use and short-cycling losses) assuming it's a post-1980 78-80% AFUE boiler with a DOE steady state efficiency of ~80% we have:

Or you could use the 99th percentile design temp of +3F, for 62 heating degrees and end up with 62 x 1550= 96,100 BTU/hr

So yeah a boiler with either an input or output of 275K is going to be on the order of 2.5-3x oversized, which is enough to affect shoulder-season efficiency, but at least it it's not 14x oversized and operating way-down the steep cliff on the part-load efficiency curve the way it would be if the measured heat load actually WAS 20K, and not ~90K.

If you're boiler is a crusty antique complete with crumbling asbestos insulation and a 25 year old retrofit conversion burner that replaced the oil burner which had been mounted on the door for the coal feed, you can safely assume it wast burning at a much lower efficiency, call it 55% instead of 80%, and convert accordingly, but you're probably looking at 60-65K for a heat load even under those circumstances.

Which is a bit on the high side for even a 3 story with 4000+ ' of living space. If it's a 3 story balloon-framed structure with little or no wall insulation the stack effect drive of a tall house will make for high infiltration rates. Retrofitting either cellulose (any density) or dense-packing one of the new school micro-fine fiberglass blowing wools (eg Optima or Spider) to 1.8lbs density would reduce that drive considerably while reducing conducted heat losses out of the walls. Air sealing the top floor ceiling/attic interface (blower door verified) and the foundation sill/band joists etc to block whole-house stack effect can also make a huge difference. If you have a flue-chase that runs unobstructed from the basement to the attic that can be a HUGE infiltration driver. Every house that vintage has some obvious and some less-obvious thermal bypasses like that, and the taller the house, the more it's actually costing you.

I'm guessing the natural infiltration on this building is well over 1 air exchange per hour, but it probably wouldn't cost an arm and leg to get it under 0.3ACH/nat, at which point the insulation can actually work. (A well insulated wind-tunnel is still expensive to heat.) An insulation contractor that does air-sealing as a service who uses blower doors and infra-red cameras to find & fix all the thermal bypasses would likely be able to cut your heat load at design temp by more than 1/3 for far less money than installing a mod-con boiler that would cut your fuel use by as much.

But if your boiler is one of those asbestos-crusted coal-to-oil-to-gas converted beasts from 1905 you may want to do both, using a Manual-J type calc for the post-air-sealed/insulated condition to size the boiler. The heat exchanger passages on the fire side in those ancient boilers are big enough to send a small Cocker Spaniel through, and it's true steady-state efficiency may be under 50%. But if it's a post-1980 boiler in decent shape it can still be hitting 80%+ steady-state, and with the Intellicon & 2-3x oversizing it'll be getting over 70% as-used, maybe even over 75%.

First , thank you VERY much for taking the time to look over my numbers, I was hoping you would chime in.

You hit on some very pertinent points to my situation.

1.) The current boiler is not ancient, its a 70's era SlantFin in very good shape just oversized.
2.) The house is very drafty and most likely lacking insulation on any exterior walls we have not done renovation on. The attic has blow in cellulose in between the joists, however the third floor is pretty much under the roofline, meaning no "true" attic above the living space.
3.) There is chimney effect going on. I have several "chases" that run the basement up to the third level that I can address.
4.) I plan on insulating the rim joists (either spray foam, or rigid foam/spray combination) before I finish the basement space.

I picked up a 2003/4 Vintage Viessman Vitogas Boiler conventional, 85.6% a couple years back and have yet to put into service.
It is a 105K BTU unit. It has sophisticated controls on it was only used at a vocational school for training on the Viessman controls. I intend on installing this unit, before any new boiler (assuming I can validate and confirm my Heatloss numbers). What kind of fuel/energy saving do you think is realistic, assuming I don't do any tightening of the house?

Here in Minneapolis we make a living converting conventional boiler with condensing boilers in 100 year old three story houses. This conversion often results in fuel savings from 25 to 50%. The return on investment easily exceeds replacement windows and even insulation as the fuel efficiency is often doubled and the system efficiency is improved with better distribution and the benefit of built-in weather sensitive controls (outdoor reset) which, lowers the operating temperatures and improves comfort at once. This is a particularly good strategy if the wall of your house can not be insulated as is so often the case with old Victorian home here in Minneapolis and so many older homes in large urban areas.

The old SlantFin probably has a DOE efficiency of 78-80%, and with 3x oversizing for the load you're looking at something like ~75%AFUE using the Intellicon control. With an appropriately-sized 86% boiler you'll probably get at least 83% AFUE out of it, so you're looking at something like ~12-15% reduction in fuel use. An appropriate sized mod-con would buy you twice that.

If the installation of the barely-used boiler is cheap or nearly free it may be worth it, spending the rest of what you might have spent on the mod-con on more serious air sealing & insulation efforts. The whole system needs to be assessed when downsizing that much though- cast iron baseboards are great for low temp operation, but if they're sized for a 275K boiler and you're replacing it with a 105K boiler there may be enough baseboard & radiator out there to make the return water to the boiler a bit too cool, cool enough to cause condensation on the heat exchangers. This can be protected with near-boiler plumbing (bypasses, etc). You can't just vent a mid-efficiency boiler into an oversized flue. The cooler stack temps and lower overall BTU output means you'd be springing for a (narrower, metal, probably stainless) flue liner to keep condensation from ruining the flue. Sketch out the design and costs of what it takes in some detail before diving in to be sure it's worth it. Excess cast iron baseboard is GOOD for condensing efficiency, and side-venting a mod-con is cheap. (Whatever you do, don't scrap the cast iron for fin-tube on any remodels- cast-iron baseboard is far better for comfort, efficiency, and low-temp operation. The big-old high-mass radiators are too, but they can be replaced with thinner profile flat-panel radiators if need be.)

If you're finishing the basement it's also well worth putting at least R8 of foam on the foundation wall and building out a batt-insulated 2x4 studwall for the finish walls (unfaced batts only, no interior vapor retarder.) In a 3 story with a full basement air sealing the basement is more important from a fuel-use point of view than insulating the walls, but insulating the walls still counts.

Replacement windows are often the least cost-effective solution for dealing with century-old double hungs. Top of the line low-E triple-track storms plus tightening up and re-weatherstripping the old windows is higher performance/lower cost than replacement windows. In buildings that tall the windows are the least of the infiltration issues- it's the stack effect that's driving it.

If it's a flat roof or simple low-pitched gable/no-attic situation, the next time the roof is done putting R20 roofing iso (3.25" thick) above the roof deck would allow you to seal the attic venting, which helps sealing the top of the stack big-time. Even partition walls on the top floor can be significant air leaks into the attic (especially if they were built with out top plates, as was popular for awhile), but unless you have sufficient R above the roof deck to keep the roof deck warm enough to not take on moisture in winter you can't just seal up the attic ventilation. (IRC 2009 calls out R20 above the roof deck for unvented attics for this climate zone, but more is always better.)

The new boiler will be installed by yours truly so costs are limited to some parts. If I did a mod-con I would probably vent out of the wall so as to not have to deal with lining my HUGE and VERY tall chimney.

The cast-iron radiation can prob be run at a lower temp, with the exception of 2 rooms that would need to have more radiation added (fintub convecors under the windows).

There are no plans to replace the windows, but this summer I will begin the room by room refurb and weatherstripping of the existing double hungs. I am keeping the original windows.

Roof pitch is pretty steep prob 6 or 8 pitch and was done 5 years ago with a "lifetime" roof.

I was considering adding staple up radiant from on the entire first level that would be able to be fed with low temp water. From reading previous posts my biggest efficiency gains are reached for every 10degree drop in boiler water.

The new boiler will be installed by yours truly so costs are limited to some parts. If I did a mod-con I would probably vent out of the wall so as to not have to deal with lining my HUGE and VERY tall chimney.

The cast-iron radiation can prob be run at a lower temp, with the exception of 2 rooms that would need to have more radiation added (fintub convecors under the windows).

There are no plans to replace the windows, but this summer I will begin the room by room refurb and weatherstripping of the existing double hungs. I am keeping the original windows.

Roof pitch is pretty steep prob 6 or 8 pitch and was done 5 years ago with a "lifetime" roof.

I was considering adding staple up radiant from on the entire first level that would be able to be fed with low temp water. From reading previous posts my biggest efficiency gains are reached for every 10degree drop in boiler water.

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With an 86% efficiency boiler you usually have to line the chimney anyway, and even at 50'-60' of height it's a given that the existing flue is too big for a 105K burner. I'm not sure if it's allowed, but at the very least you'd be looking at inserting some type of liner. Ideally you'd want the flue to be on the order of 5" in diameter or less, even at 50' to maximize flue velocity, minimizing flue condensation. If your's is a big old 12 x 12" or 8 x 12" terracotta liner (or unlined brick) it might have still done OK with an 80% 275KBTU/hr burner, since it's more heat (buoancy) and a higher initial stack temp. But cutting the BTU input by 2/3 and starting with a lower stack temp you can be guaranteed to get condensation in an oversized flue. The closer you are to the max-BTU rating of the flue size, the less likelihood of flue condensation. A 5" round flue (~20 square inch cross section) is near-ideal, but a 6" (28" cross section) would still be OK. Too much bigger than that you can think of the flue as being the distillation coil for condensing the exhaust, which WILL ruin the masonry over time (faster with oil than with gas, but still with gas nonetheless.)

If adding radiation under the windows, use cast iron baseboard or the thin-profile 3" convecting radiators that look totally appropriate in a house that age. Brand new it's kinda pricey, but there are Craigslist deals on cast iron baseboard every day in southern New England at $20/foot or less (eg: http://newhaven.craigslist.org/mat/2782166241.html ). It cleans up an re-paints nicely too. Fin-tube tin gets kicked around and beat up pretty easily, and at 120F or less the output is very non-linear, whereas cast iron keeps on going even at 90F water temps, and you need a sledgehammer to put a mark in it.

The real efficiency gains from radiant floors and other low-temp radiation isn't linear with temp. At above 130F on up you get about a 3% reduction in fuel use for every 10F you can drop the temp, and at ~130F RETURN water temp combustion efficiencies are around 87%. But when the return water drops much lower there's a sharp knee in the curve as the combustion exhaust starts condensing on the heat exchangers in the boiler, giving up it's "latent heat" or "heat of vaporization". The improvement rises pretty rapidly dropping from 130F to 110F, and more slowly thereafter, but is also firing rate dependent:

If you have sufficient radiation that your OUTPUT temp need not be above 130F even at design-condition loads, with outdoor reset control you'll be in the high-90s for combustion efficiency, and that's pretty commonly met with better radiant floor designs in a CT climate. (It may take extruded aluminum heat transfer plates or above-the subfloor type systems, and probably wouldn't cut it with suspended tube or thin-plate under the subfloor.) But since you don't need anything like peak design water temps at the average winter heat loads, outdoor reset gets you into the condensing range most of the time with radiators and cast-iron baseboard too, even if it may not be condensing for those pre-dawn hours on days where it hits the outside design temperature.

With the 86% boiler there are still efficiency benefits to running outdoor reset on the system water, plumbing the boiler with protection to ensure that the return water entering the boiler stays above 130F. At 180F boiler output the distribution losses to the basement etc are high, even with insulated plumbing, and unless that basement is insulated, very little of that loss accrues to the first floor zone. The Intellicon on the existing boiler is doing a bit of that for you already by killing the burner when it determines that the thermostats will soon be satisfied, purging heat from the boiler into the system to end the cycle, and inhibiting the burner on initial calls for heat until the heat-purge has reached the programmed minimum temp. Being that oversized heat-purging the boiler for lower standby & distribution loss with the Intellicon will perform as-good or better than outdoor reset, but with the new boiler, maybe not. Comfort is enhanced with outdoor reset schemes, but electricity use for pumping power goes up a bit.

Do the windows have exterior storm windows? If not, storm windows (particularly low-E storms) are a VERY cost-effective way of cutting the heat load in your climate. On the south facing windows you can probably get more benefit from clear glass (higher solar gain) than low-E versions, as long as it's a pretty-tight model. It's worth paying a bit extra for the low-E storms on the other sides of the house though. The cost/benefit of replacing existing storms is less clear- depends on their condition (particularly air-tightness.)

Yup- you'll never get the kind of BTU/ft requirement that seems to be needed at design temp out of anything but plated subfloor systems. (Although we don't really know the available square footage). If the true heat load is 90K, at 15BTU/ft^2 it would take 6000' of floor. (If it's more like 60K, it would still take 4000' of floor.) But at 25BTU/ft @ 60K design load 2400' would make it and condensing temps would be most of the time.

With outdoor reset you could still get some condensing performance out of an un-plated staple up in CT- the average winter loads just aren't that severe. Mean January temps are in the mid to high 20s in the interior, low 30s in coastal towns.

I have a "supply" of base-ray that I got from a co-worker, when his father passed. Probably about 60' ft or so if i had to guess. I LOVE this stuff. It was installed probably in the 40-50's to replace the full size cast iron radiators, but they only did the 1st and 2nd floor, and for whatever reason they didn't use them in the living room and dining room. The third floor still has the regular cast iron rads.

Funny side note: My mom lives with us and has a mobility scooter. She crashes into the castiron baseboard radiators probably 5-10times per day. If these were fin-tube copper they would be GONE. The CI baseboard stuff takes a lickin and keeps on tickin

The first floor that I would "consider" doing staple up on would be about 1500sq ft. I also would not use the underfloor radiant to replace the existing stuff, but more to supplement it. The living room and dining room have the underwindow convectors, six in total. 4 of which are copper-fin tube style and 2 of which are actually a "castiron" setup inside of the enclosures. It would be nice to have warm floors in the kitchen and foyer that are tiled. Not looking for toasty, but just not the ICE cold floors they are now(even with socks on)

The Viessman 85% boiler has outdoor reset on it's controller and boiler protection. It is equipped with a Vitotronic 200/300 I think.

The chimney is probably 40' and I don't know the lining. It has 2 flues, one for the Fireplace and another for the existing boiler. You are probably right that it is oversized.

I have tripple track aluminum style storms on the upper story and no storms on 1st and 3rd floor.

Inherently self-protecting boilers are a good thing (the Burnham Revolution series are similarly protected with the internal bypass plumbing & controls, necessary to achieve that on-the-hairy-edge-of-condensing 88% efficiency). I haven't searched out all of the relevant documents on the Viessman, but it sounds like you could make out pretty well with it. I'm not sure if this note on venting is relevant to the vintage model you have but it probably is:

If yours doesn't have an integrated automatic vent damper an isn't power-vented, get one- your tall chimney literally sucks (air), and unimpeded it'll drive a lot of infiltration. At least the stack effect of the actual stack has a useful function, but it's useful only while the burner is on. It's even raising the air-conditioning load if it has no flue damper. If it is power vented, it may be cheaper & easier to side-vent it (and brick up the flue entry for the old boiler.) At my niece's place we side-vented ~70KBTU ~85% Burnham PVG selected for avoiding the cost of lining existing flue for the more appropriate sized boiler in her Worcester MA 2.5 story. We could have gotten away with a bigger, ~82% efficiency boiler using the existing flue but it would have been oversized for the load and about the same money as the tiny 3-plate forced draft unit.

The behemoth steam boiler it replaced was about 8x oversized for the heat load, if fine for the existing radiation. It was in fact exactly the "...crusty antique complete with crumbling asbestos insulation and a 25 year old retrofit conversion burner that replaced the oil burner which had been mounted on the door for the coal feed..." I had in my mind's eye when writing that line. The burner-tech who had tested it on the yellowing decades old tag on the gas conversion burner had claimed 75% efficiency, but I doubt it was anywhere near that even on day-1, given the size of the fire-side passages in that hulk- it had plenty more dilution air/excess combustion air than the burner itself was providing, resulting in an unrealistically low stack temp.

Shop hard for the best performance on storm windows, and be willing to spend a bit for the super-air-tight versions, and for low-E (they're not always easy to find.) With leaky windows on the first & third floors that could end up being significant stack effect driver after you've fixed all the rest. Read through some of the backup documentation (particularly the National Lab testing) linked to on this site. If you need them to match the existing triple-tracks you'd still get a performance boost by replacing them, but nowhere near what you get by going from single-pane double-hungs to a high-performance storm. Even the most expensive storm window will be cheaper than a low-end replacement window, and would even outperform all but the best double-pane replacement windows.

It's worth doing a room-by-room Manual-J type heat loss calc to get the zone balances right. Even with the cheapo sheet metal heat transfer plates you can get a lot of heat into the room at low water temps, and if it's supplemented by c.i. baseboard you could get it to run at condensing temps most of the time if you wanted to. Breaking it up into ever more micro-zones isn't advisable, so getting the whole 1500' to run at a balanced temp requires a bit of design work, with or without the radiant floor.

Air sealing the basement and insulating the foundation can raise the temp of the first floor a bit and would avoid the ice-cold feel on the coldest winter morning, but it's NOTHING like having radiant floor underfoot from cruisin' in yer socks comfort point of view. There's no payback to radiant floors from a net-present-value on future utility savings point of view, but from a human comfort point of view it's "worth it" in this climate, in any place you plan to live for more than a few years.